The Linux IPL Procedure SHARE - Tampa February 13, 2007 Session 9274 Edmund MacKenty Rocket Software, Inc.
Purpose De-mystify the Linux boot sequence Explain what happens each step of the way Describe why each step exists Tell you how to learn more DASD
General Design Principles Flexibility: uses not thought of by designers Extensibility: accommodate specific end-user needs Reuse-ability: of code and user data Controllability: higher-level code can drive it Portability: can operate in different environments Simplicity: easy to understand, use; limited side-effects
Overview Boot loader Kernel Initial RAM disk Init process Runtime configuration scripts User login
Concepts The Kernel Device drivers Kernel modules Filesystems Mounting a filesystem Processes The onion The two trees Run Levels The online manual
Manual Pages Online manual is a good source of information References to manpages use the form: page(section) Section 1: User Commands Section 2: System Calls Section 3: Library Functions Section 4: Special Files Section 5: File Formats Section 6: Games Section 7: Conventions and Miscellany Section 8: Administrative Commands To learn about init(8), use the command: man 8 init Use info(1) for more information about some commands The appropos(1) and whatis(1) commands do searches Different distros have different manpages available
Linux: The Onion Linux consists of many layers surrounding a kernel
Linux: The Two Trees Linux consists of two trees: Processes and Files Processes inherit properties from their parent Files reside within their parent directory
Structure of the Kernel The Linux kernel is not monolithic All device drivers and many sub-components may be built as modules, which can be loaded or unloaded as needed. This permits one kernel to run efficiently on lots of different hardware. The kernel build process is amazingly configurable. Some core components must be compiled in: Memory management Virtual filesystem layer Process scheduler Multi-processor support TCP/IP networking (if used) Examples of dynamically-loaded modules: Filesystems: ext3, reiserfs, jfs Support for specific hardware: SCSI, DASD, USB, Crypto Network drivers
Processes A unit of execution scheduled by the kernel Each process runs in its own address space Fork: creates a new, child, process Inherits code and data segments Gets copies of all open files, sockets, etc. Process execution returns from fork() call Exec: Loads a new program into a process All open files are closed New code and data segments are allocated Process execution continues at entry point of new program Running a program means a process forks and the child execs the program Process A fork() Process B exec() Process B
Filesystems A logical structure built within a disk partition to manage files Many kinds of filesystems are supported There is one root filesystem: the base of the directory tree A filesystem of any type may be mounted on a directory Mounting is how new storage devices are added Unreferenced filesystems may be unmounted / home root /bin /home /lib /usr... /fred /mack /nms usr /lib /local /X11 bin home lib usr fred mack nms lib local X11
The Boot Loader z/vm IPLs a Boot Loader from DASD zipl(8) is the boot loader for zseries Linux Knows where to find the kernel within the Linux filesystem Passes kernel command-line options Configured in /etc/zipl.conf [zipl.conf(5)] Uses the eckd0 program to store the subchannel address Reads kernel file into memory, jumps to entry point DASD zipl Memory Kernel
Starting the Kernel Kernel is usually in a compressed file Beginning of file is program that uncompresses the rest Kernel builds its memory pools Kernel detects processors, estimates their speed Kernel starts its internal threads Kernel initializes build-in device drivers Drivers do hardware detection Drivers can use kernel command line arguments
The Initial RAM disk What is an initial RAM disk, and why use one? Extra drivers and setup code Useful when entire kernel won't fit on a floppy (for x86) Lets a distro have a single kernel config across all platforms On zseries, initrd loads the DASD device driver Boot loader told kernel where to find initrd Kernel creates a temporary filesystem in memory Unpacks the initrd image into that filesystem Runs the program /linuxrc on it
Initial RAM disks for zseries Loads kernel modules DASD device driver ext3 filesystem LVM drivers Does LVM initialization [see lvm(8), vgscan(8)] Mounts the real root filesystem from DASD Makes the real root filesystem be the system root The mkinitrd(8) tool creates the initrd image
Finishing Kernel Initialization Kernel continues when /linuxrc on the initrd ends Makes the root filesystem read-only, so it can be checked Finds /sbin/init and runs it
Init: process number one Init(8) is the first user-mode process It is the root of the process tree All other processes are started by init or its descendants Reads its configuration file: /etc/inittab [see inittab(5)] Invokes rc-scripts [see init.d(7)] Manages changes between runlevels
Example inittab file (SuSE SLES 8) # The default runlevel is defined here id:3:initdefault: # First script to be executed, if not booting in emergency (-b) mode si::bootwait:/etc/init.d/boot # /etc/init.d/rc takes care of runlevel handling l0:0:wait:/etc/init.d/rc 0 l1:1:wait:/etc/init.d/rc 1 l2:2:wait:/etc/init.d/rc 2 l3:3:wait:/etc/init.d/rc 3 #l4:4:wait:/etc/init.d/rc 4 l5:5:wait:/etc/init.d/rc 5 l6:6:wait:/etc/init.d/rc 6 # what to do in single-user mode ls:s:wait:/etc/init.d/rc S # what to do when CTRL-ALT-DEL is pressed ca::ctrlaltdel:/sbin/shutdown -r -t 4 now ~~:S:respawn:/sbin/sulogin /dev/ttys0 # on S/390 enable console login in all runlevels 1:012356:respawn:/sbin/mingetty /dev/ttys0
What is an rc-script? Runtime configuration scripts live in /etc/init.d Each rc-script manages a distinct service or daemon These are shell scripts (but they don't have to be) Each accepts a single command as an argument: start: starts the service, initializing some resource stop : stops the service, shutting down some resource restart: stops then starts the service status: tells you what state the service is currently in
What Is A Runlevel? A feature of the init(8) program Controls which processes are allowed to run Change to runlevel N with command: init N Runs master rc-script (/etc/init.d/rc) with new runlevel Stops all rc-scripts not in the new runlevel Starts all rc-script that are in the new runlevel Runlevels are implemented by directories containing symbolic links to rc-scripts (/etc/rc?.d) KXXname stops (kills) the service named name. SXXname starts the service named name.
Traditional Set Of Runlevels 0: Halt the system 1: Single user mode 2: Multi-user mode 3: Multi-user with networking 4: (unused) 5: Multi-user with networking and graphical desktops 6: Reboot
Boot-time rc-scripts Run at boot-time from /etc/init.d/rc via init(8) Bring up user-space (non-kernel) resources: Mount /proc and /sys pseudo-filesystems (kernel interfaces) Check the root filesystem [fsck(8)] Initialize the LVM subsystem, searching for devices using LVM [vgscan(8)] Check all remaining filesystems [fsck(8)] Enable any swap devices Re-mount root to be writable Mount all other filesystems as described by /etc/fstab [fstab(5)]
Service rc-scripts Initialize services and daemons for a particular runlevel Bookkeeping daemons: cron periodically run other commands hotplug detect newly-installed devices (DASD being linked) syslog collects logging output from other processes Network services: interfaces assign IP addresses or do DHCP, set up routes NFS mount network filesystems Network daemons: sendmail SMTP daemon listening on port 25 xinetd a meta-daemon listening on many ports, invokes FTP, TELNET... NTP Network Time Protocol daemon using UDP connections Applications: X-Windows Starts an X display manager to provide user desktops WebSphere Starts up a web services engine DB2 Starts one or more database instances
User Logins on Terminals Init(8) starts getty(8) processes on attached terminals Getty(8) sets up serial tty lines, auto-detecting speed, etc. Getty(8) presents a login: prompt Exec's login(1), giving it the username Login(1) gives password: prompt, does authentication If successful, login(1) invokes the user's shell init fork()/exec() getty exec() login exec() bash
User Logins from the Network An rc-script starts sshd(8) processes. Sshd(8) listens on port 22 for network connections Sshd(8) forks a child process to handle each connection SSH client negotiates user credentials with server Child sshd authenticates the user credentials If successful, child sshd forks the user's shell Child sshd continues to encrypt/decrypt data with SSH client init fork()/exec() fork()/exec() /etc/rc sshd fork() sshd fork()/exec() bash
The Linux IPL Procedure Contact Information: Presenter: Company: Ed.MacKenty@RocketSoftware.com http://www.rocketsoftware.com